The present application claims priority to Japanese Application No. P11-012216 filed Jan. 20, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to an optical disc having a light transmitting layer on a substrate/and adapted for recording and/or reproducing information signals by illuminating laser light from the side of the light transmitting layer.
2. Description of the Related Art
As an optical disc, such a one has so far been widely known in which a replica substrate of light-transmitting plastics, having irregularities on one of its major surfaces, is provided, a reflective film or a recording film is provided on the major surface thereof having the irregularities, a protective film is formed on the signal recording surface and in which the laser light is illuminated from the side of the substrate surface, that is from the side of the substrate opposite to the signal recording surface, to record and/or reproduce information signals.
In such optical disc, it is demanded to reduce the substrate thickness, in order for the substrate to perform the role of a light transmitting layer, if the large capacity exceeding 8 GB is be taken into consideration.
However, the substrate of the optical disc is usually molded by injection molding, such that there is necessarily a limitation in reducing its thickness.
If, in fabricating a substrate 120 mm in diameter, transcription properties of the irregularities are to be secured at an ordinary level (level of a conventional optical disc), the substrate thickness cannot be reduced beyond approximately 300 xcexcm. On the other hand, if the irregularities are to be transcribed with sufficient accuracy to cope with the increasing capacity, the substrate thickness cannot be reduced beyond approximately 500 xcexcm.
For this reason, it is extremely difficult to fabricate by injection molding an optical disc substrate, with a thickness of the order of 100 xcexcm, on which micro-sized irregularities have been transcribed with high accuracy.
This is due to the problem inherent in injection molding, for example, sporadic flowing states in the injection material in a metal mold, that is molten resin, sporadic cooling speed of the metal mold, that is variations in the temperature or viscosity of the molten resin, and so forth.
In view of the above-described status of the art, the present Assignee proposes an optical disc in which a light transmitting layer not larger in thickness than 500 xcexcm, more specifically, not larger than 177 xcexcm, is formed on a signal recording surface of the substrate, and in which the laser light is illuminated from the side of the light transmitting layer to record and/or reproduce the information signals.
In this optical disc, the substrate can be of an ordinary thickness, such as, for example, 1.2 mm or 0.6 mm, so that the irregularities can be transcribed with high accuracy by injection molding. On the other hand, the laser light for recording or reproduction is illuminated from the side of the light transmitting layer with a reduced thickness, thus allowing to accommodate high recording capacity.
Meanwhile, if the laser light is illuminated from the light transmitting layer for recording and/or reproduction, the manner of forming the light transmitting layer poses a crucial problem, because fluctuations in the thickness of the light transmitting layer affect the characteristics significantly.
Although it may be contemplated to apply e.g., a UV curable layer by, for example, a spin coating method, for constituting a light transmitting layer, it is difficult in this case to form the light transmitting layer to a uniform film thickness.
It is therefore an object of the present invention to provide an optical disc having a light transmitting layer of a thin uniform thickness.
In one aspect, the present invention provides an optical disc in which a transparent film is bonded via a pressure-sensitive adhesive sheet or a dry photopolymer sheet on a substrate and in which a signal recording surface is formed on the substrate and/or the transparent film, wherein recording and/or reproduction for the signal recording surface is effected from a light transmitting layer comprised of the pressure-sensitive adhesive sheet or the dry photopolymer sheet and the transparent film.
In another aspect, the present invention provides an optical disc in which at least one major surface of a substrate is a signal recording surface and in which a pressure-sensitive adhesive sheet or a dry photopolymer sheet is bonded as a light transmitting layer on the signal recording surface, wherein recording and/or reproduction is performed from the light transmitting layer with respect to the signal recording surface.
According to the present invention, there may be provided an optical disc which has a reduced uniform thickness to permit higher density recording than is possible with the conventional system.
If, in particular, pressure-sensitive adhesive sheet is used as an adhesive layer, the optical disc may have a high light transmission factor over a wide wavelength range. This is particularly meritorious if high density recording by short wavelength recording is to be achieved.